Enable Headers interface part of fetch API (#463)

crates/web-sys/tests/all/headers.rs

@@ -0,0 +1,33 @@
+use super::websys_project;
+
+#[test]
+fn headers() {
+    websys_project()
+        .file(
+            "src/lib.rs",
+            r#"
+                #![feature(proc_macro, wasm_custom_section)]
+                extern crate wasm_bindgen;
+                use wasm_bindgen::prelude::*;
+                extern crate web_sys;
+
+                #[wasm_bindgen]
+                pub fn test_headers(_headers: &web_sys::Headers) {
+                    // empty for now...
+                }
+            "#,
+        )
+        .file(
+            "test.js",
+            r#"
+                import * as assert from "assert";
+                import * as wasm from "./out";
+
+                export function test() {
+                    let headers = new Headers({'Content-Type': 'text/plain'});
+                    wasm.test_headers(headers);
+                }
+            "#,
+        )
+        .test();
+}

crates/web-sys/tests/all/main.rs

@@ -2,6 +2,7 @@ extern crate wasm_bindgen_test_project_builder as project_builder;
 use project_builder::{project, Project};
 
 mod event;
+mod headers;
 mod response;
 
 fn websys_project() -> Project {